In the production of printed circuit boards, the surface of copper is treated to promote the adhesion between the copper surface and a resist before coating the copper surface with a dry film of etching photoresists, solder resists, etc. In the treatment of substrates carrying fine wiring pattern, the chemical etching is usually used. In the production of multi-layered printed circuit boards, it has been attempted to promote the adhesion between a copper electroconductive patterned layer and a resin layer, for example, by forming an oxide layer on the copper surface and reducing the oxide layer to metallic copper by a reducing agent while maintaining the geometric shape of the oxide layer.
A negative pattern of the circuit is formed by a) applying an etch resist, e.g., a polymeric dry film resist or a metal resist on a layer of copper, b) etching away those portions of copper not covered by the etch resist, and c) removing the etch resist from the remaining copper circuit.
Etching solutions applied for this task are selected from different types of compositions such as mixtures of an oxidising agent and an acid. Two main types of etching solutions are based on an acid such as sulphuric acid or hydrochloric acid and contain as oxidising agent hydrogen peroxide, copper ions or ferric ions. Such etching solutions are disclosed in C. F. Coombs, Jr., “Printed Circuits Handbook”, 5th Ed. 2001, Chapter 33.4.3, pages 33.14 to 33.15 and Chapter 33.4.5, pages 33.17.
The ongoing miniaturization of circuits in terms of line width/interline-space values and thickness of the copper layers to be etched does not allow to using conventional etching solutions such as the ones described above.
The disadvantage of known etching solutions is even more present if the copper tracks are manufactured by a semi additive process (SAP). Here, the bare dielectric substrate is first coated with a seed layer serving as an electrically conductive layer. The seed layer comprises for example copper deposited by electroless plating. Next, a patterned resist layer is formed on the seed layer and a thicker, second copper layer is deposited by electroplating into the openings of the patterned resist layer onto the seed layer. The patterned resist layer is stripped and the seed layer in between copper tracks deposited by electroplating needs to be removed by a differential etch step. The seed layer deposited by electroless plating has a finer grain structure than the second copper layer deposited by electroplating. The different grain structures can lead to a different etching behaviour of the individual copper layers.
A similar situation is present when copper tracks are manufactured by a modified semiadditive process (m-SAP) or advanced modified SAP (Am-SAP) wherein a thick, second copper layer is deposited in the openings of the patterned resist layer onto a first thin layer of copper. The first copper layer is manufactured, e.g. by thinning a copper clad attached to the dielectric substrate. Again, both first and second copper layer have a different grain structure.
The etching solution applied for the differential etching step should only remove the first copper layer in-between the copper tracks while not (substantially) attacking the sidewalls and the top of the copper tracks deposited by electroplating and the underlying first copper layer or copper seed layer.
Etching solutions based on sulphuric acid and hydrogen peroxide lead to an undesired undercutting of the first copper layer during etching (FIG. 1b) which results in an insufficient adhesion of the copper layer on the dielectric substrate. WO 2010/016562 discloses compositions for copper etching containing polyvinylpyrrolidinones (PVP) in conjunction with ferric and copper ions as oxidising agent. The use of PVP in such copper etching compositions also causes undercuts (see comparative Application Example 2b).
Etching solutions based on sulphuric acid and ferric ions typically show an etching behaviour as shown in FIG. 1c. This trapezoidal line shape is undesired because the broader base of the etched copper line can lead to circuit shorts which are not acceptable. This phenomenon of forming trapezoid etching results is referred to herein as “line shape alteration”.
A further undesired side effect of copper etching is the reduction of line width in general. This is typically caused by too strong etching dissolving copper ions from all surfaces of the treated copper lines (see FIG. 1d).
EP 0855454 A1 teaches cationic polymers to be used in copper etching solutions. Such cationic polymers include inter alia amine bearing polymers such as polyethylenimines. Polyethylenimines containing etching solutions, however, alter the line shape and reduce the line width of treated copper lines resulting in irregular line shapes (see comparative Application Example 3c and 3d). This is highly undesired as it impairs the electrically conductive layout of electronic appliances causing inter alia shorter life-times of such goods.